Mitigation and Deterrence Methods to Reduce Marine Mammal Interactions in Longline Fisheries for Tuna

The expansion of pelagic longline tuna fisheries, along with increasing fisheries observer coverage, has resulted in an increased documentation of interactions with small cetacean marine mammals. In particular, western North Atlantic fisheries interact with short- and long-finned pilot whale and Rosso's dolphins, while the central North Pacific fisheries interact with false killer whales. Other species infrequently also interact with this gear type, including bottlenose and common dolphins. Small cetacean interactions include depredation of caught fishes, hooking events, and entanglements.

Although some seasonality and area effects occur with small cetacean interaction rates, such effects are generally weak, and time-area closure discussions for these fisheries have met with strong opposition within U.S. constituent-based take reduction teams. A variety of alternative mitigation methods have instead been examined, including restrictions on the lengths of pelagic longline sets, limited soak times, and changes to baits. Another strategy involves requiring vessels to move a selected distance away from the location of any interaction. More recently, investigations into so-called “weak hook” technology, which exploit the weight difference between the target tunas and the bycatch small cetaceans, have been underway in the central North Pacific and the western North Atlantic. All of these strategies will be discussed within the context of reducing small cetacean interactions with pelagic longline fisheries.